Color transforms modify color data delivered to a reproduction device to modify a nominal response of the reproduction device. Exemplary reasons for doing so include: establishing an intended tonal response for one or more color channels, establishing an intended reproduction of selected colors, and establishing a device response that simulates or matches a different device condition.
Color transforms can be one-dimensional, as exemplified by tonal correction transforms, which transform values for a single color channel (e.g. C1→C2) based on a one-dimensional curve or LUT. Color transforms can also be multi-dimensional, as exemplified by color matching profiles, which transform color coordinate vectors comprising values for multiple color channels (e.g. [C1,M1,Y1,K1]→[C2,M2,Y2,K2]) based on a multi-dimensional curve or LUT. In some cases, multiple sets of transforms can be successively applied for different purposes, as described in U.S. patent application Ser. No. 12/014,821.
Color transforms can be automatically generated using tools that measure or otherwise acquire information about the nominal and intended response of the reproduction device. However, these methods can be time consuming and may require the measurement of multiple reproductions produced by a device. Multi-dimensional transforms are also conceptually complex and typically require computation of a significant number of data points to yield desirable results.
One-dimensional transforms, in contrast, are conceptually simpler and computationally easier to produce but can still be time consuming to produce if reproduction measurements must be made. In addition, changes to one-dimensional transforms can affect the color response of a device so that it no longer matches an intended color response. For example, adjusting a tone reproduction curve for one color channel of a device may adversely affect the reproduction of neutral colors by the device by introducing a color cast or by changing the uniformity of a neutral color ramp.
In some situations, such as during a press run, the actual device response may appear different than intended. This may require an immediate adjustment that precludes the use of measurements to produce the desired transform. For example, a green cast may appear in the neutral color ramp (i.e. the device coordinates that are supposed to produce perceived neutral colors). As another example, reproduction of a neutral color ramp may be compressed in one region so that the perceived gradation in neutral colors reproduced is not as expected (e.g. uniform throughout the ramp).
If time is of the essence, manual creation or editing of existing transforms may be the only practical method for correcting perceived mismatches between intended and actual device response. Manual adjustment of multi-dimensional transforms is simply too complex to be reliably done. Instead, manual adjustment of one-dimensional transforms is typically the preferred method for handling such problems. Of course, there may be other situations when manual editing of one-dimensional curves is desired, when timeliness is not the prime motive but rather the problem to be resolved is most readily solved by editing one-dimensional transforms.
However, manually adjusting one or more one-dimensional transforms may affect the overall color response of the device and thus require additional iterations of adjustments to compensate. In many cases, the complexity and/or time required to achieve the ideal adjustment is too great and a compromise is made. For example, a color cast may be reduced but not entirely removed. As another example, a cast may be removed but neutral gradation uniformity is sacrificed. As another example, neutral gradation uniformity is improved but a color cast is introduced. As another example, a color cast may be reduced in one region but a different cast is introduced in another region.
Automated methods, such as those disclosed in the related application, can predict the impact to color response from changes to one-dimensional transforms. This information can be used to guide a user to iteratively make adjustments that minimize the undesirable effects of the adjustment without making reproductions. However, the number of iterations may require excessive time. Thus, a means for quickly creating and/or editing one-dimensional color transforms to effect a change in a nominal device response is required, without iterative adjustments or additional device response measurements.
As indicated above, color transforms are also useful for modifying data so that one device condition can emulate the response of another device condition. As an example, image data intended for a first device condition, characterized by a smaller gamut, can be modified for a second device condition, characterized by a wider gamut. An emulation goal can be to match color amongst reproductions made by both device conditions. In this case, multi-dimensional transforms, such as a device link, can be created.
An alternative goal can be to make the images similar but take advantage of the wider color gamut (e.g. richer saturated colors). In this case, a device link can also be created but a set of one-dimensional transforms may be preferred for a number of reasons. For example, one-dimensional transforms are easier to comprehend, easier to compute and easier to edit. As another example, one-dimensional curves are guaranteed to map the surface of one gamut to another while the use of interpolation in processing multi-dimensional transforms may cause certain portions of one gamut surface to map to the interior of the other gamut.
Prior art methods exist for device condition emulation by creating one-dimensional transforms, but these methods typically involve iteratively adjusting data used to create the transforms to minimize color errors (e.g. in the interior of the gamut) for a selection of device coordinates. Thus, a similar need exists to create one-dimensional emulation transforms, without the need for iterative adjustments or additional device response measurements.